In a word, ebay.
There are several for $35 NIB and also auctions. That is how I picked up mine, "on the cheap".
And I always count my mags in my range bag after shooting! Lesson learned!
Extra mags are very helpful after hand loading different loads. For example pretty normal for me to have 125 grain in one mag and 200 grain in another.
The lowest paid college major/degree in this country after graduation...
Now, go figure...